Here are 2 shots of the big Sanxian and small Sanxian lying together:
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The big Sanxian measures: 123cm x 22cm(diameter of resonator) and the small Sanxian measures 100cm x 15cm.
You can imagine the medium Sanxian to be, well, somewhere in between.
Here are pictures and a sample clip of the big Beijing Rosewood Sanxian:
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And here's the small Shanghai Rosewood Sanxian again:
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Pardon the less than perfect playing. I had to put a gun to his head to make him play.